Increasing demand for transceiver devices with small form factors has motivated research on highly integrated low-cost transmitter and receiver circuits. Direct conversion transmitter and receiver circuits can simplify the signal path by translating a desired radio frequency (RF) spectrum to a zero intermediate frequency (IF) via a single mixing stage or circuit, and thus effectively eliminate image frequency problems and hence expensive and bulky off-chip image-reject filters required in conventional heterodyne receivers. It also allows for channel selection to be performed at baseband with a simple low-pass filtering prior to a high dynamic analog-to-digital conversion. Therefore, the direct conversion approach can offer highly integrated, low-cost, low-power and multi-standard solutions e.g. for wireless products. However, several problems such as direct current (DC) offset still exist in direct conversion architectures. To solve these problems main efforts have been put on factory calibration, thus increasing testing time and hence costs.
An alternative approach to avoid DC offsets is to employ a sub-harmonic mixer (which may also be called harmonic mixer), where the local oscillator (LO) frequency operates at an integer division of the RF input frequency. The specific case of an integer division of “2” is also referred to in the literature as even-harmonic mixer (EHM). The sub-harmonic mixer functions transform a desired RF frequency into baseband, while rejecting the fundamental LO frequency. In practice, sub-harmonic mixer functions can be implemented by passive or active circuitry, wherein both configurations require an additional 45-degree phase shift in LO signal generation to obtain both in-phase (I) and quadrature phase (Q) channels in baseband.
On the other hand, in digital video broadcast (DVB) receivers there is a demand to reject interferences at 3rd order and 5th order harmonics of the desired carrier frequencies, which interferences are introduced by strong transmission signals of 2nd generation (2G) and 3rd generation (3G) wireless applications and cannot be completely filtered out by RF filters. Therefore, a harmonic-rejection mixer is necessary for enabling co-existence of mobile television and cellular applications.
Similar to direct conversion receiver architectures, direct up-conversion is increasingly used in transmitter architectures, due to its high level of integration. As the LO frequency is equal to the carrier frequency, intermediate up-conversion employed in dual up-conversion architectures can be eliminated. Therefore, the image problem is no longer present and discrete RF filters with high quality factors Q are not required before the power amplifier (PA). However, the direct up-conversion architecture suffers from a so-called LO pulling problem. Despite various shielding techniques employed in transmitter design, the strong PA output may still be coupled to a voltage controlled oscillator (VCO) circuit which generates the LO signal, and may thus corrupt the LO spectrum (which is called “LO pulling”). The phenomenon of LO pulling is alleviated if the interference is far away from the LO frequency. Usually, a VCO circuit with frequency two times the carrier frequency is thus used, followed by a divide-by-two circuit. The LO pulling problem can thus be solved by doubling the VCO frequency.
Unfortunately, another mechanism called VCO remodulation still exists if the PA needs to be integrated with the RF transceiver into the same chip, for example in a Bluetooth application. Then, the second harmonic of the PA output will modulate the VCO circuit, resulting in RF spurs at the carrier offset of three times the IF input frequency. In principle, the VCO frequency cannot be an integer multiple of the carrier frequency, and vice versa. A possible solution is to resort to a very complex LO generation scheme where the VCO frequency is a fraction of the carrier frequency, such as for example 4/3.